The isomers of xylene find wide and varied application. For example, meta-xylene (m-xylene) is used in the manufacture of dyes, and ortho-xylene (o-xylene) is used as a feedstock for producing phthalic anhydride, which finds use in the manufacture of plasticizers. However, currently the most valuable of the xylene isomers is para-xylene (p-xylene), since p-xylene is a feedstock for terephthalic acid, which in turn is used in the manufacture of polyester fibers and films.
The majority of p-xylene produced today is derived from crude oil via reforming of the naphtha portion of the crude into a mixture of benzene, toluene, and xylenes (BTX) and heavier aromatics. These aromatics then undergo a variety of reactions, such as transalkylation, disproportionation and xylene isomerization, to increase the concentration of p-xylene. The current commercial process also requires extraction of aromatics from non-aromatics and separation of p-xylene from a mixture of xylene isomers via crystallization or molecular sieve adsorption. The overall process is therefore complex. Moreover, as crude oil prices rise, so does the feed stock price for p-xylene production via the current commercial routes. Recently, as crude prices have risen, the prices of coal and natural gas having fallen making syngas derived from these sources cheaper on a carbon or energy equivalent basis and a potentially attractive feed for the production of basic chemicals, such as p-xylene.
The conversion of syngas to olefins and paraffins has been widely practiced for many years via the Fischer-Tropsch process and indirectly via the methanol to olefins (MTO) process. However, both of these syngas conversion routes produce only small amounts of aromatics and there is therefore a need for an improved route for converting syngas to aromatics and particularly p-xylene.
In a paper entitled “Direct Conversion of Syngas into Aromatics over Bifunctional Fe/MnO—ZnZSM-5 Catalyst”, Chinese Journal of Catalysis, Volume 23, No. 4 July, 2002, Wang Desheng et al. report that syngas can be converted into aromatics at high yield using a bifunctional catalyst comprising Fe/MnO mixed with Zn-ZSM-5 containing up to 7 wt % Zn under conditions including a temperature of 517° F. (270° C.), a pressure of 1100 kpa (absolute), and a hydrogen to carbon monoxide molar ratio of 2:1. However, the aromatic product slate obtained in the process of Wang et al. is composed mainly of benzene and toluene, rather than the more desirable p-xylene. There is, therefore, an ongoing need to provide a process of converting syngas to aromatics in which the yield of xylene isomers, and in particular p-xylene, is improved.